Process for the catalytic synthesis of amines



Nov. 9, 1937. c. H. GREENEWALT 2,093,289

PROCESS FOR THE CATALYTIC SYNTHESIS OF MINES Filed July 13, 1934 MEOH STORAGE NH; STORAGE MONO-AMINE STORAGE TRl-AMINE STORAGE NG TANK EX PANS VALVE 1 PUMP WASTE GAS OUT TO SCRUBBER HEAT EXCHANGER COOLER CONDENSER.

PREHEA KECTI FYING CATALYST CHAMBER NH FREE AMINES TO STOKAGE 0k OTHER SEPARATION EQUIPMENT INVENTOR. Crawford H. Greeneu/cc/z BY QM QMZ/W ATTORNEY.

Patented Nov. 9, i937 PROCESSFOR THE CATALYTIC SYNTHESIS OF AMINES Crawford H. Greenewalt, Wilmington, Del llsignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application July 13, 1934, Serial No. 734,928 ln'Great Britain October 13, 1932 14 Claims. (Cl. 260127) This invention relates to catalytic processes reactants from the pump. The cooled reaction for the synthesis of alkyl amines from alcohols products are further cooled and then are passed and ammonia and more particularly to the prointo a rectifying column where the ammonia is duction of methyl amines from methyl alcohol separated as a liquid fraction at the top of the and ammonia. column and the ammonia-free amines are sepa- 6 This case is a continuation in part of the aprated as a liquid fraction at the bottom of the plicants copending applications, Serial No. column. The Waste gas that comes off as a 558,106, filed August 19, 1931 and Serial No. gaseous fraction at the top of the column is cooled 675,092 filed June 9, 1933. first by means of a water-cooled condenser and 1 The catalytic synthesis of amines from alcohols then by means of a brine-cooled condenser, where 10 and ammonia has been described in the literaany ammonia contained therein is condensed. ture. The reaction is ordinarily carried out at The pressure on t e Waste 8 18 then released atmospheric pressure in the vapor phase in the and the gas is passed out of the system. The ampresence of various dehydrating oxides, such as monia from the water-cooled and brine-cooled thoria, alumina, zirconia, silica, and tungsten condensers and from the top of the rectifying 15 oxides. The products which are obtained in the column is returned to the ammonia storage vapor state are either absorbed in a suitable tanks.

solvent or compressed, cooled and condensed. The following examples describe preferred The resulting solution or condensed mixture is forms of the invention. These examples are not 20 then separated, usually by distillation. to be construed as limiting the scope of the in- 20 One of the objects of this invention is to provention but are to be taken as illustrative only. vide an improved process for the catalytic syn- Example I thesis of amines from alcohol and ammonia whereby the products are obtained directly in a A liquid mixture containing 37.3% ammonia suitable onditi for separation Another and 62.7% methyl alcohol was charged into a 25 ject is the combination of synthesis and separa- 10118 cylindrical Steel Vessel P d h Sight tion steps with recirculation of the undesired glasses and connected at the e to th Sucti n products over the catalyst in such a manner Side of a a l pr portioning pump. The amthat 9, o id r bl saving in equipment and maniacal mixture in the steel cylinder was power i effected, oth objects 1 appear pumped directly into aheated reaction tube con- 30 hereinafter, taining 100 cc. of a catalyst comprising alumina These objects are accomplished by the followsupported on silica e The c a t tube was ing inventio hi h comprises cyclic process contained in an electric furnace which served to wh h i l d conducting t vapors of alcohol .maintain the catalyst temperature at 440 C.

and. ammonia, mixed in suitable proportions, over and Supply heat to a vaporizing chamber 35 an active catalyst at elevated pressures, condenslocated/ Prior the catalyst The qu d ing the products to a liquid state, separating by Products Were Condensed, Passed 111110 & pa adistillation, and recirculating over the catalyst 1 and the Pressure on the reaction system w s the undesired products together with additional maintained at 230 per gauge by Vent- 40 amounts f alcohol and m ing' the small volume of fixed gases formed dur- 40 Figure 1 1S a flow sheet Showing a pref rr d ing the passage over the catalyst. Nearly comform of the process described by the applicant plete recovery of the materials charged was ob- In the process disclosed by the fl w sheet, tained in the form of a condensed liquid prodmethyl alcohol, ammonia, monomethylamine and This Product was analyzed by Well known trimethylamine are drawn off in their various methods- The following table gives compara 45 storage tanks and mixed in a mixing tank. The tive analyses of the charging stock and of the liquid mixture is pumped to a pressure of about Product- 200 lbs. per sq. in., where it is heated and preheated so as to vaporize the various constituents. 0011mm! Reach!!! Product The vapor is then passed through the catalytic 50 chamber where the ammonia and methanol react Ammonia a to form dimethylamine. The products of reacgggggflgggfT tion in vapor form are passed from the catalytic Trimathyl amine chamber through a heat exchanger in heat exchange relationship with the incoming liquid 55 IO v The conversion of methyl alcohol to amines was 95%. In the event that all three. of these amines are not desired, the ammonia, methanol and the undesired amine or amines are separated from the desired product and recirculated over the catalysts combined with the required additional amounts of methanol and ammonia.

Example II For the purpose of preparing pure dimethylamine a liquid mixture containing 28.4% ammonia, 12.0% of monomethylamine, 39.7% trimethylamine, and 19.9% of methyl alcohol was pumped from a suitable pressure vessel through a coil of heated, pressure resisting tubing which served as a vaporizer. From the vaporizer, the mixture now at 380-390 C. and completely in the vapor state, was passed over 1 liter of a suitable catalyst containing principally alumina. The pressure vessel containing the catalyst was provided with an external heating element and the temperature in the center of the catalyst bed was maintained at 400-410 C. The reaction pressure was controlled at 230 lbs. per sq. in. gauge and' the injection rate at approximately 2.0 liters per hour. The reaction products were cooled and led into a pressure separator. The system pressure was held at a constant value by withdrawal of the small volume of fixed gas resulting from alcohol decomposition from the top of the above separator. The alcohol thus decomposed was but a small fraction of the total charged, 1. e., about The liquid products which accumulated in the separator under reaction pressure. were transferred to a batch still and fractionated. An ammonia fraction, 9. fraction containing principally monomethylamine, and a third out free from monomethylamine but containing a mixture of diand triamines were obtained. These distilled fractions were subject ed to chemical analysis and the composition of the original reaction product determined. A small amount of the reaction mixture was neutralized with sulphuric acid and the resulting salt solution distilled. No unconverted methyl alcohol was found.

The following table gives a comparison of the compositions of the charging stock and of the products as determined by distillation and analytical data. Since the losses on the synthesis and distillation steps were negligible, these figures may be compared directly.

Where the process is directed to the preparation of pure dimethylamine, the dimethylamine is separated from the product and the remaining constituents of the product are combined with the proper amounts of ammonia and methanol and recirculated over the catalyst. The presence of the undesired amines in the reaction chamber causes suppression of their formation with the final result that dimethylamine is substantially the only product. When it is desired to control the'process to obtain trimethylamine as the only product or as the principal product, a high ratio of methanol and ammonia is maintained in the reaction mixture andthe trlmethylaooazeo amine is separated from the product. The monomethylamine and dimethylamine which are formed are recirculated over the catalyst together with the proper amount of methanol and ammonia.

Example III The amine synthesis was carried out in the manner outlined in Example H, but the cooled reaction product, instead of passing to a high pressure separator, is led directly into a small rectifying column operated under the same pressure as the synthesis system. The liquid in the calandria at the base of this column is boiled with steam and reflux is supplied at the top by cooling with water. Liquid ammonia is withdrawn from the top of the column, conveyed to suitable storage facilities, and reused in the amine synthesis. The liquid in the boiler" or calandria is cooled and discharged to storage vessels or directly to equipment suitable for the isolation of the individual amines. The vent gas from the top of the water cooled condenser is passed to an ammonia cooled auxiliary condenser where its temperature is reduced to about C. This low temperature suiiices to liqueiy most of the ammonia vapor escaping the first condenser. The condensed ammonia is returned to the column as reflux. The equipment for the catalytic synthesis and for the exhaustion of unconverted ammonia from the reaction product is operated as a single unit and the pressure on the entire system, i. e., synthesis and ammonia exhaustion column, is controlled by regulation of the volume of cooling water to the ammonia stripping column condenser.

Example IV Methyl alcohol and liquid, anhydrous ammonia were pumped individually to 2000 lbs. per sq. in., passed through a coiled prehester where complete vaporization of both constituents was effected, and the vaporized mixture forced over 220 cc. of an aluminum phosphate catalyst maintained at 340 C. The rates of injection were 178 g. and 112 g. per hour for methyl alcohol and for ammonia respectively. The products were cooled and expanded to 1 atmosphere into a scrubbing column. The expanded products passed up through the scrubbing column counter-current to a dilute acid solution which reacted with the basic constituents and carried the resulting salts to a receiver at the base of the column. The fixed gases, such as carbon monoxide and hydrogen, were vented from the top of the scrubbing column and metered. The salt solution was analyzed by methods well known to those familiar with the art, and it was found that 71.5% of the methanol charged was converted to methylamines. Of the methyl alcohol fed to the catalyst chamber, 20.0% was recovered as monomethylamine, 26.2% as dimethylamine, and 27.7% as trimethylamine.

Reaction pressures of 230 and 2000 lbs. per

sq. in. gauge have been mentioned in the above densation of ammonia with ordinary cooling water. This pressure, of course, will vary with seasonal fluctuations in cooling water temperature but will generally be within the range 175-300 lbs. per sq. in. gauge. The gases passing into the catalyst may be preheated to a temperature somewhat lower than that of the reaction chamber and the catalyst should be heated to between 300 and 500 C. in order to have favorable reaction conditions. The. temperature, however, will vary with the type of catalyst used and the rate of flow oi. the gases over the catalyst. It is within the contemplation vof this invention, however, to use any temperature at which amines will be formed when the gases containing ammonia and alcohol are brought into contact with the catalyst.

My invention is applicable to the simultaneous production of all three methylamines from methyl alcohol and ammonia, and to the synthesis of any desired amine individually by separation of the desired amine from the reaction product recirculation of the other two amines as disclosed for operation at atmospheric pressure in United States Letters Patent, Serial No. 1,799,722, issued April 7, 1931 to H. R. Arnold. The concentration of reactants for the case in which all three amines are to be produced depends upon the characteristics of the products which are desired, and may be varied at will without affecting the operation of. my process.

If monomethylamine is not desired as an end product but is to be recovered and recycled to prevent its further formation, it is feasible to remove both ammonia and the mono derivative as heads" by fractionating the mixture, and thus leave the diand triamines for further processing. Should the proportions of reactions contacted with the catalyst be such that the reaction product contains little or no ammonia, it is advantageous to proceed in the general manner outlined in Example III but to strip monomethylamine from the reaction mixture.

My invention is not limited to the use of a specific catalyst but can be successfully carried out with a number of active dehydrating materials. Among these may be mentioned alumina on charcoal and silica gel, basic aluminum sulphate, and aluminum phosphate.-

Ammonia and methylamines are extremely volatile bases, and the best practical method of separating a mixture of the above materials is to subject it to a pressure distillation. Methylamines, when produced by synthesis at substantially atmospheric pressure, are obtained largely in the vapor state and must be compressed or refrigerated prior to fractionation under pressure. An important advantage of my invention is the elimination of a gas compression operation between the synthesis and separation steps. A material savings in power is effected, since it is much cheaper to pump the liquid reactants to the synthesis pressure than to compress and liquefy the gaseous products.

It is especially advantageous to combine the synthesis and initial distillation steps in a single operation maintained at essentially the same pressure throughout. This reduces equipment and labor costs, simplifies control problems, and permits the production of ammonia free methylamines at a uniform rate. Intermediate compression and absorption operations are avoided.

In addition to the above advantages, it has been found that the rate of catalyst productivity is materially increased by operation at elevated pressures. This decreases catalyst requirements and equipment size.

As many apparently widely diiferent-embodimerits of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended patent claims.

I claim:

1. A process of making lower alkyl amines, which comprises catalytically synthesizingthe amines at a pressure in excess of. 100 lbs. per

square inch and then directly fractionating the reaction product at substantially the same supcratmospheric pressure to separate the components.

2. A process of making lower alkyl amines, which comprises passing a vapor comprising essentially ammonia, and a lower alkyl alcohol in proportions sufficient to react to form amines over an amination catalyst at a temperature within the range of 300 to 500 C. and cooling the resulting vapors, while maintaining the system under a pressure in excess of 100 lbs. per square inch sufilcient to liquefy the vapors in the cooling step, said pressure being substantially the same in both the catalytic step and in the cooling step.

3. A process of making lower alkyl. amines, which comprises passing a vapor comprising essentially ammonia and a lower alkyl alcohol in proportions sufficient to react to form amines over an amination catalyst at a temperature within the range of 300 to 500 C. and cooling the resulting vapors, while maintaining the system under a pressure in excess of 100 lbs. per square inch suflicient to liquefy the vapors when cooled to room temperature, said pressure being substantially the same in both the catalytic step and in the cooling step.

4. A process of making lower alkyl amines, which comprises passing a vapor comprising essentially ammonia and a lowen alkyl alcohol, in proportions suflicient to react to form amines over an amination catalyst at a temperature within the range of. 300 to 500 C. and cooling the resulting vapors, while maintaining the system under a pressure in excess of 100 lbs. per square inch sufficient to liquefy the vapors in the cooling step, and while maintaining that pressure constant by periodically bleeding the vapor system, said pressure being substantially the same in both the catalytic step and. in the cooling step.

5. A process of making methylamine, which comprises passing a vapor comprising essentially ammonia and methyl alcohol in proportions sufficient to react to form amines over an amination catalyst at a. temperature within the range of 300 to 500 C. and cooling the resulting vapors, while maintaining the system under a pressure in excess of 100 lbs. per square inch suflicient to liquefy the vapor in the cooling step, said pressure being substantially the same in both the catalytic step and in the cooling step.

6. The process of claim 5, in which the pressure is maintained at 100 to 400 lbs. per square inch.

'7. The process of claim 5, inwhich the pressure is maintained at 175 to 300 lbs. per square inch.

8. A process of making ower alkyl amines, which comprises the step of pa sing a vapor comprising essentially ammonia and adower alkyl alcohol in proportions suflicientto reacttaform amines over an amination catalyst at a pressure of about .230 lbs. per square inch and at a temperature sufflcient to cause the conversion of the said alcohol and ammonia to amines, cooling the resulting products to liquefy same and passing them directly to a rectifying column under the same pressure as the system to separate from the amines formedunreacted ammonia together with any trimethylamine that may come over as an azeotropic mixture with the unreacted ammonia.

9. A process of making amines, which comprises passing a vapor comprising essentially a lower alkyl alcohol and ammonia in proportions sumcient to react to form amines, under a pressure of 200 to 400 lbs. per square inch and at a temperature sufllcient to cause the conversion of the said alcohol and ammonia to amines, over an amination catalyst, cooling the reaction product and rectifying same, while maintaining the system under the pressure initially created.-

10. A process which comprises creating a pressure of between and 400 lbs. on a liquid mixture containing essentially ammonia, monomethylamine, trimethylamine and methyl alcohol, heating said liquid mixture to vaporize same,

preheating the vapor, passing the preheated vapor over an amination catalyst heated to between 300 and 500 C., cooling the reaction product and rectifying same, while maintaining the system under the pressure initlally created.

11. A process which comprises heating a liquid comprising essentially ammonia, monomethylamine, trimethylamine and methyl alcohol, so as to vaporize same, preheating the vapors to between 380 and 390 C passing the preheated vapors over an amination catalyst heated at 400 to 410 0., cooling the reaction products so as to liquefy any amines and ammonia present, while maintaining a pressure in the system of about 230 lbs. per square inch.

12. A process of making dimethylamine, which comprises creating a pressure on a liquid mixture consisting of approximately 28% ammonia, 12% monomethylamine, 40% trimethylamine and 20% methyl alcohol, of about 230 lbs. per square inch, heating said liquid mixture to vaporize same, preheating the vapor to about 380 to 390 0.. passing the preheated vapor over an amination catalyst heated to about 400 to 410 C.; cooling the reaction products, passing the cooled products into a rectifying column and thereby separating from the amines formed the unreacted ammonia in the form of an azeotropic mixture of ammonia and trimethylamine, cooling this ammonia-trimethylamine azeotrope to condense same, while maintaining the entire system under a constant pressure of about 230 lbs. per square inch, separating the monomethylamine and trimethylamine from the amines formed and recycling these two amines together with the ammonia to help make up the initial liquid mixture.

13. In a recirculatory catalytic process of making a lower alkyl amine from ammonia and a lower aliphatic monohydric alcohol in which the formation of the undesired amine is inhibited by separating the desired amine and recirculating the undesired amine over an amination catalyst, the steps which comprise maintaining the entire system at a pressure in excess of 100 pounds per square inch, compressing the reactants to the pressure of the system and introducing same into the system at a point so that they will pass over the catalyst, and fractionally distilling at the pressure of the system the product obtained by the contact of the reactants with the catalyst to separate the desired portion of the product from the undesired.

14. Process in accordance with claim 13, characterized in that the lower aliphatic monohydric alcohol is methyl alcohol.

CRAWFORD H. GREENEWALT. 

